When drilling for oil and gas, a wellbore or borehole of an oil or gas well is typically drilled from surface to a first depth and lined with a steel casing. The casing is located in the wellbore extending from a wellhead provided at surface or seabed level, and is then cemented in place. Following testing and other downhole procedures, the borehole is extended to a second depth and a further section of smaller diameter casing is installed and cemented in place. This process is repeated as necessary until the borehole has been extended to a location where it intersects a producing formation. In some cases, a final section of tubing known as a liner may be located in the wellbore, extending from the lowermost casing section or casing ‘shoe’ to the producing formation, and is also cemented in place.
In order to access oil or gas containing formation outside the casing, the casing may be perforated at a selected zone by means of explosives. The perforated casing optionally may be opened up further by appropriate tooling to form an enlarged hole or slot referred to as a “window” in the art. Alternatively, a milling tool may be used for the purpose of removing a substantial section of casing to provide a window to permit drilling of a side track through the casing to provide access to another region of formation or to effect a by-pass or deviated wellbore. Such a milling operation results in large amounts of swarf which has to be circulated out. Similarly where a milling operation is required to remove a broken or stuck pipe or tool or any other junk that is obstructing normal operations, large amounts of metallic swarf have to be recovered.
Where relatively smooth bore casing or liner defines the outer surface of the annulus around the drill string the return of metal swarf can be achieved and the recovery of such metal swarf may be enhanced by use of magnetic tool elements incorporated in the string.
However, when circulation of such swarf-laden fluids is to pass through a blow out preventer (B.O.P.) which has cavities or recesses housing contingency equipment, passages allowing an intervention or coupling of ancillary equipment, a potential risk factor is realised in that each cavity, recess or passage represents a trap for metallic swarf debris. Where the metallic swarf fouls in such cavities, recesses and passages, the consequences may be that operational use of contingency measures can be compromised. In particular concerns arise in relation to the integrity of the sealing surfaces of the B.O.P. rams. It will be understood that such risks arise at any point in the fluid circulation path and could impact on B.O.P. performance whether it is located at the wellhead on the seabed or at surface e.g. below the drill floor. However such risks are exacerbated when the B.O.P. is located at depth, i.e. at the wellhead on the sea floor, where access is restricted and there is a high dependency on reliability of the contingency measures provided thereby.
The B.O.P. is a critical contingency measure for re-gaining control of wellbore pressure when it exceeds controlled circulation fluid pressure and a high degree of confidence is required that it will operate when required without failure.
Currently, some operators prefer to avoid the considerable risk associated with such a fouling of the B.O.P. by retrieving the B.O.P. after such a milling operation to conduct an inspection. When the B.O.P. is recovered to surface for such an inspection, and then subsequently re-deployed it is not unusual to lose from 1 to 2 days in turn around. The time involved in recovery inspection, and re-running of the subsea B.O.P. to the wellhead for re-installation leads to a high cost which currently appears unavoidable to a prudent operator.
However, cost is not the dominating factor, and the decision to pull the riser and subsea B.O.P. stack is not one to be taken lightly in view of the considerable HSE risks associated with completing the operation. It will be understood that the stack is an assembly of contingency measures weighing several hundred tons, that a deep water marine riser also weighs several hundred tons and that the controlling vessel typically has to take account of dynamic loadings and risks to failure of joints and seals in the combined lift package. Handling of the retrieved B.O.P. and riser components involves additional risks to personnel. Therefore, there are numerous HSE risks associated with these heavy lifting operations.